1. Field of the Invention
The present invention relates to a magnetic head that records signals on a magnetic recording medium, and a magnetic recording device.
2. Related Art
In recent years, magnetic recording devices such as hard disk devices have been rapidly becoming smaller and having higher densities, and this trend is expected to continue in the future. To achieve higher density in magnetic recording, it is necessary to narrow the recording track width to increase the recording track density. It is also necessary to increase the recording density in the longitudinal direction of the track, which is the line recording density.
However, as the recording density becomes higher in in-plane longitudinal recording, a larger diamagnetic field is generated. This leads to a decrease in reproduction outputs, and hinders stable recording. To solve those problems, vertical recording methods have been suggested and put into practical use. By a vertical recording method, recording is performed by magnetizing a magnetic recording medium in a direction perpendicular to the film plane. By such a method, the influence of a diamagnetic field is made smaller even when the recording density is increased, and higher recording density can be achieved, compared with a case where a longitudinal recording method is utilized.
A magnetic recording head that can be used in a magnetic disk device of a vertical recording type is of a single-pole type or a shielded-pole type, and is different from a ring-like head that is conventionally used in in-plane recording. A magnetic recording medium to be used in this case is a double-layer magnetic recording medium having a soft-magnetic backing layer.
Conventionally, an AMR head that utilizes anisotropic magnetoresistance effects (AMR) or a spin-valve GMR head that utilizes giant magnetoresistance effects (GMR) and has higher sensitivity has been used to reproduce signals recorded on a magnetic recording medium. Also, magnetic heads that utilize tunnel magnetoresistance effects (TMR) or CPP (Current Perpendicular-to-the-Plane)-GMR elements and have higher sensitivity are also being developed and put into practical use. In each of those magnetic heads, a sense current is applied in a direction perpendicular to the film plane.
Meanwhile, the following phenomenon has been observed. When TMR elements or CPP-GMR elements having spin-valve structures are made smaller in size and have a certain amount or more of current applied thereto, the magnetization of the free layer of each element has ferromagnetic resonance, and oscillates at a fixed frequency equal to or higher than 1 GHz. With the use of this phenomenon, spin torque oscillators of several GHz to several tens of GHz have been suggested (see U.S. Patent Application Publication Nos. 2005/0023938 and 2005/0219771, for example).
Also, the following high-frequency assist recording has been suggested. In this high-frequency assist recording, a high-frequency magnetic field generated by each spin torque oscillator is utilized in an auxiliary manner, so as to enable writing with a small recording magnetic field.
However, in a case where a high-frequency magnetic field generated by each spin torque oscillator is used in an auxiliary manner, the high-frequency assist magnetic field needs to be generated at a portion closer to the write (trailing) shield from a region near the main magnetic pole of the recording head. Therefore, spin torque oscillators are formed in the magnetic gap of the recording head. If an actual recording operation is performed, the operations of the spin torque oscillators become unstable due to the magnetic flux that are short-circuited and flow from the main magnetic pole to the write shield. As a result, a sufficient high-frequency magnetic field for assisting recording operations cannot be generated.